1. Field of the Invention
This invention relates to a machining system wherein a single common master controller is used for the operational control of a plurality of numerical control (NC) machine tools. More particularly, this invention relates to a machining system wherein an operation mode selector is provided in the machine controllers of each NC machine tool and which is suitable for efficient machining of metal molds.
2. Description of the Prior Art
In general, metal molds are produced in small lots and are of complex shapes. Therefore, it is very difficult to prepare NC programs for metal mold machining, and the metal molds are, in many cases, machined with a profile machine tool. Further, since the material of workpieces used for the production of metal molds is hard and difficult to cut and the workpiece shapes are complex, the path of a tool for cutting the workpieces must be sharply changed at many points. Accordingly, the operator must watch for tool breakage, excessive cutting resistance, etc. in operating the machine tool. For these reasons, it is difficult to automate and systemize the machining of metal molds.
Recently, however, automatic units for preparing NC programs for the machining of complicated, three-dimensional, curved surfaces have been developed. Also developed have been various automatic functions, such as for changing abnormal tools detected by an automatic broken tool sensor and a tool life monitor with a spare tool; a machining condition monitor allowing under optimum machining conditions; and an automatic pallet changer for automatically changing workpieces. These developments have been placing automation of metal mold machining with NC machine tools in closer reach.
On the other hand, it has also been proposed to employ a group control system for machine tools in which a plurality of NC machine tools and a workpiece carrying line connecting these NC machine tools are operated and controlled by a single central processor in the absence of operators. This system is effective for small and medium lot production. For example, U.S. Pat. No. 3,576,540 discloses a system for operating and controlling a plurality of machine tools, a workpiece carrying line, and a tool carrying line by a single computer. U.S. Pat. No. 4,069,488 discloses a method for minimizing the processing time of the central processor in a system for controlling and monitoring many machine tools.
The conventional group control system mentioned above is designed for mass production of workpieces having a relatively simple shape and made of easy-to-machine material. Unmanned machining is relatively easy for these workpieces. Further, in this system, the central processor, the NC machine tool, and the workpiece carrying line are specially designed to suit the system. Direct application of this conventional system to metal mold machining would bring about very real problems, as described below.
Namely, in NC machining of metal molds, since the shapes of the metal molds are complicated, the NC unit must quickly receive and process many blocks of NC machining information. In the conventional system, the limited signal transferring capacity of the central processor used in the system makes it practically impossible to quickly send many blocks of NC machining information from one central processor to the NC unit of every NC machine tool without passing through machine controllers for intermediate processing. Accordingly, if the conventional system is directly used for metal mold machining, the NC machining information cannot be sent to the NC units close upon each other. As a result, the tool feed operation is interrupted during the process of metal mold machining, resulting in ridges, usually referred to as cutter marks, on the machined surface of the workpiece and a longer machining period.
Further, since the machining sequence made according to the production plan and recorded in the central processor cannot easily be changed, this system cannot quickly cope with unscheduled rush machining of workpieces with machining sequences not recorded in the processor and cannot cope with changes in production plans. Also, since the conventional system is composed of specially designed NC machine tools and workpiece carrying line, it is impossible to incorporate existing general-purpose NC machine tools into the system.
Furthermore, unlike mass production, orders are usually for production of a single metal mold. Therefore, it is economically impossible to conduct trial machining and is essential for the operator to monitor the machining condition. Thus the operator must sometimes interrupt the NC machining operation to inspect the machining condition. However, since the conventional system is designed for unmanned machining, it is difficult to obtain access to the workpiece being machined. Moreover, it is difficult for the operator to stop the operation of the machine tools of this system at a desired point of time for the purpose of inspection and then to resume the machining from the point of interruption.